Re: SR cannot determine Contraction

Dono wrote:
On Feb 25, 9:47 pm, Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote:
The "pole and barn" gedanken is valid. [...]

This is not what I asked you. What I asked is can the pole fit in the
barn with both doors closed simultaneously in the barn frame?

With the parameters I gave, or with the parameters you gave, yes.


In other words, do you , Tom Roberts, believe that uniform relative
motion makes objects shorter?

That's your confusion. This is phrased so poorly that no answer is possible. Your phrase "makes objects shorter" implies a change to the object, but observations made from another frame cannot possibly affect the object itself.

Uniform motion makes objects be measured to be shorter than when they are at rest. And if the "measurement apparatus" includes doors that close simultaneously in their frame, then for appropriate values of the parameters a moving object can fit between closed doors that are closer together (in their rest frame) than the proper length of the object -- at least in principle.

Remember how the length of a moving object is measured: its front and rear locations are marked SIMULTANEOUSLY in the measuring frame, and then the distance between the marks is measured with rulers at rest in the measuring frame. But the order can be reversed: one can pre-position doors an appropriate distance apart, and "mark" the ends of the object by opening/closing them. See my description of the gedanken without accelerations, and my solution to the problem you posed.

As I have said before, IMHO the pole and barn gedanken is best understood as displaying the relativity of simultaneity, rather than length contraction.


Tom Roberts
.

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